1. Field of the Invention
The present invention generally relates to lithography, and more particularly to systems and methods for inspecting an object (such as, a reticle or wafer) of a lithography system.
2. Background Art
Lithography is widely recognized as a key process in manufacturing an integrated circuit (IC) as well as other devices and/or structures. A lithographic apparatus is a machine, used during lithography, which applies a desired pattern onto a substrate, such as onto a target portion of the substrate. During manufacture of ICs with a lithographic apparatus, a patterning device—which is alternatively referred to as a mask or a reticle—generates a circuit pattern to be formed on an individual layer in an IC. This pattern may be transferred onto the target portion (e.g., comprising part of, one, or several dies) on the substrate (e.g., a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (e.g., resist) provided on the substrate. In general, a single substrate contains a network of adjacent target portions that are successively patterned. Manufacturing different layers of the IC often requires imaging different patterns on different layers with different reticles. Therefore, reticles must be changed during the lithographic process.
To ensure that the pattern is properly transferred to the target within appropriate tolerances, the reticle and/or the substrate (e.g., silicon wafer) on which the IC is printed may be inspected for defects or other characteristics. An object (e.g., reticle or wafer) can be inspected by collecting light scattered off or transmitted through fine structures on the surface of the object. A specially designed objective typically directs the light toward the object and collects the scattered or transmitted light from the object. The amount of information about the fine structures on the object depends on the spectral bandwidth of the light and the numerical aperture (NA) of the objective. Increasing the spectral bandwidth of the light and the NA of the objective, increases the amount of information that can be collected by the objective. Therefore, wide spectral bandwidth and high NA objectives are desired. From a manufacturing perspective, however, wide spectral bandwidth and high NA objectives are problematic because the objective should reduce chromatic aberrations (axial color) caused by the wide spectral bandwidth and reduce obscurations caused by the high NA.
In general, three classes of objectives may be used to collect information about an object (e.g., reticle or wafer): (i) an all refractive objective; (ii) an all reflective objective; or (iii) a catadioptric objective. Although all refractive objectives may not have a central obscuration, these types of objectives typically do not adequately correct chromatic aberrations (axial color) caused by the wide spectral bandwidth at DUV wavelengths. In addition, there is a limited number of refractive materials that can transmit high energy electromagnetic radiation (such as, deep ultraviolet (DUV)), further constraining the types of all refractive objectives that can be manufactured with desirable characteristics. Accordingly, all refractive objectives are not desirable for object-inspection purposes.
Unlike an all refractive objective, all reflective and catadioptric objectives can adequately correct chromatic aberrations (axial color). This is because reflective surfaces are apochromatic (i.e., reflective surfaces can reduce chromatic aberrations by combining three colors to a single focus). Unfortunately, conventional, rotationally-symmetric all reflective and catadioptric objectives typically have a central obscuration. Any obscuration is undesirable because it reduces the amount of collected light—and therefore the amount of information that can be collected about the fine structures of the object (e.g., reticle or wafer). Although it may be possible for an all reflective objective to be configured without a central obscuration, these types of all reflective objectives are typically not rotationally symmetric, resulting in undesirable size and packaging constraints. More importantly these all reflective objectives will have high-NA limitations. Accordingly, like all refractive objectives, all reflective objectives are not desirable for object-inspection purposes.